Tuning electron viscosity with light

A team of researchers at the Indian Institute of Science (IISc) and NIMS, Japan, has recently developed a technique that enables them to tune the defect density in ultraclean hexagonal boron-nitride-encapsulated graphene devices, thereby controlling the flow of electrons in such devices. This work highlights a pathway to dynamically switch between fluid-like and disorder-dominated transport within the same device.

They demonstrated that the flow of electrons near room temperature can resemble a viscous liquid – a hallmark of electron hydrodynamics – and can be reversibly and controllably tuned using ultraviolet (UV) illumination, combined with an applied back gate electric field. UV light activates transient charge-trap states within the dielectric layers and interfaces, allowing them to modulate momentum-relaxing scattering rate in the graphene without permanently degrading the channel.

To understand this phenomenon, the team used electrical transport techniques in conjunction with Johnson-Nyquist thermometry, a highly sensitive noise measurement technique used to determine electronic temperature and thermal conductivity. It is reported that devices violate the Wiedemann-Franz (WF) law in their pristine condition at room temperature and approach the recovery of the WF law when subjected to UV irradiation as back gate voltages are applied progressively.

REFERENCE:
Gugnani A, Majumdar A, Watanabe K, Taniguchi T, Ghosh A, Dynamically tunable hydrodynamic transport in boron-nitride-encapsulated graphene, Physical Review Letters (2025).

https://journals.aps.org/prl/abstract/10.1103/z38z-dz5l

LAB WEBSITE:
https://physics.iisc.ac.in/~arindam/